In recent years, in accordance with the arrival of a multimedia era in which information, services, entertainment, communications and the like are united, need for transmitting data of a large data amount at a high speed is progressively increasing, and it is also requested that bipolar transistors can operate at a higher speed. In order to enable the bipolar transistor to operate at a higher speed, a width of a base layer has to decrease and its carrier concentration has to increase. However, when a base layer is formed by implanting ions of impurities, due to channeling of impurities caused when ions of impurities are implanted, it is difficult to realize the base width of less than 40 nm. Therefore, it has been considered to deposit a base layer on a silicon (Si) substrate by an epitaxial growth method.
However, even when the homogeneous-junction base layer is deposited on the substrate by an epitaxial growth method, if a carrier concentration of the base layer increases, then holes that are to be implanted into the emitter from the base increases so that a current gain decreases. For this reason, there is considered a hetero-junction bipolar transistor in which a base layer containing a silicon germanium (Si1-xGex, hereinafter referred to as a “SiGe”) film having a band gap width narrower than that of the Si film is deposited on a single crystal Si substrate by an epitaxial growth method, injections of holes into the emitter being considerably decreased by utilizing a fact that a potential barrier relative to holes is higher than a potential barrier relative to electrons.
In the hetero-junction bipolar transistor, a base resistance can be decreased by increasing a carrier concentration of the base, and a sufficiently large current amplification width (hFE) can be obtained. As a result, while a sufficiently large withstand voltage is being maintained, a high frequency characteristic can be realized.
However, since Si and Ge have different lattice constants and thermal expansion coefficients, a stress occurs in the base layer containing SiGe. As a consequence, during a manufacturing process of heterojunction bipolar transistor after the base layer has been formed, a crystal defect which is referred to as a “misfit dislocation” occurs in the base layer so that a yield of the hetero-junction bipolar transistor is lowered. Because the misfit dislocation strongly depends on the Ge concentration in the SiGe, if the Ge concentraton is lowered, then it is possible to suppress the occurrence of the misfit dislocation. However, according to the above-mentioned proposal, effects for narrowing the width of the band gap of the base layer also decrease so that a hetero-junction bipolar transistor having desired performance cannot be obtained.
As another countermeasure for suppressing the occurrence of such misfit dislocation, it has been reported that the use of silicon germanium carbon (Si1-x-yGexCy, hereinafter referred to as a “SiGeC”) in which a very small amount of carbon (C) is added to SiGe is effective (“CVDSi1-x-yGexCy, epitaxial growth and doping control”, pp. 32–37, Subcommittee of The Society of Applied Physics, Silicon Technology No. 18, 23rd Jun. 2000). This technique is based on the fact that a distortion caused by a difference between lattice constants of Si and Ge can be alleviated by adding C to SiGe.
Moreover, since SiGeC has a considerably high diffusion control effect for impurities such as boron (B) as compared with SiGe, SiGeC has another advantage that the hetero-junction bipolar transistor can obtain a steep B doping profile having a high concentration as compared with SiGe. This is effective for decreasing a base resistance (Rb) and reducing a base running time (τB) and which is advantageous for improving a high frequency characteristic of the hetero-junction bipolar transistor.
However, when the base forming region is prescribed by a silicon oxide film formed on a substrate, if an SiGeC film that serves as a base layer communicating with a single crystal Si substrate exposed through an opening of this silicon oxide (SiO2) film and the SiO2 film is formed, then a stress that is caused by a difference between thermal expansion coefficients of the SiGeC film and the SiO2 film occurs so that the misfit dislocation tends to occur.
Therefore, it is difficult to provide a semiconductor device containing a hetero-junction bipolar transistor at a high yield.